Ethylene polymerization



p 1956 D. c. PEASE ETAL 2,762,791

E'I'HYLENE POLYMERIZATION Filed Sept. 24. 1955 A. LIQUID PHASE ETHYLENEYM VAPOR PHASE ETH E POLYMER I MOISTURE PERMEABILITY ACTUAL MOISTUREPERMEABILITY U II II H B PERCENTAGE COMPOSITION A INVENTOR DONALD C.PEASE MILTON J. ROEDEL ATTORNEY States Patent Orifice 2,762,791 PatentedSept. 11, 1956 ETHYLENE roLYMEmzATroN Donald C. Pease and Milton J.Roedei, Wilmington, Del, assignors to E. I. du 'Pont de Nernours andCompany, Wilmington, 'Del., a corporation of Delaware ApplicationSeptember 24, 1953, Serial No. 382,056 18 *Claims. ('01. 260-94.9)

This invention relates to a process of polymerizing ethylene to highdensity polymers and more particularly to the polymerization of ethylenein an improved reaction environment. This application is acontinuation-in-part of copending applications S. N. 98,198, filed June10, 1949, and S. N. 248,445, filed September 26, 1951, which in turn isa continuation of now abandoned application S. N. 98,197, filed June 10,1949.

It is known that ethylene can be polymerized under various conditionswith the aid of such catalysts as oxygen, persulfates, dialkylperoxides, azo compounds, and the like. All of these prior processesemploy highly compressed gaseous ethylene, alone or in admixture withorganic or inorganic liquids, and temperatures of 40 C. and above. Suchconditions are commercially feasible, but because :of the high pressuresemployed require costly equipment. Also, the olymer obtained under theseconditions, while more resistant to moisture than most other polymers,still has a moisture permeability which is undesirably high for manypurposes. This is believed to be due, at least in part, to the branchedchain structure and possibly to the high amorphous content.

An object of the present invention accordingly is to provide a processfor polymerizing ethylene which aYQids the need for vusing costly, highpressure equipment and which produces ethylene polymers possessing ahigh degree of linearity, high density, high degree of crystallinity,and which forms films possessing a high degree of moistureimpermeability. Another object is to provide new catalysts for thepolymerization :of ethylene. Still another object is to provide areduction-oxidation procass for the preparation of solid polymers ofethylene in a reaction medium containing an oxidizing agent and an ionof a polyvalent metal in one of its lower valence states. Other objectsand advantages of the invention will hereinafter appear.

The above and other objects of the inventionare realized by coolingethylene below itscritical temperature of 9.6 C. under suificientpressureto liquefy, the ethylene, and then polymerizing the .ethylene toa solid polymer in liquid ethylene as a reaction medium. For mosteffective operation of the process, the polymerization is carried out inthe presence of an ethylene olymerization catalyst and under variousother conditions hereinafter speeified.

Useful catalysts for the process include the metal alkyls, the aliphaticazo compounds of Hunt U. .S. 2,471,959, issued May 31, 1949, pe'roxygencompounds, and other compounds which yield reactive free radicals below9.6- C. The activity of the metal alkyls-isimproved by certain metals,viz., copper, silver, gold, iron, c obalt, and nickel, or their salts.The activity oflthe per oxygen compounds is improved by silverions or.byions ofpne'or more polyvalent metals of atomic number 2 2 ,to 29, in-

clusive (titanium, vanadium, ch romium manganese, iron,

cobalt, nickel, and copper) in their lowerfstate of oxidation, ferrousions being preferredflfpr 'econon'iic and other reasons. The polyvalentmetal i on r'nay"ei ther introduced inthe lower stateuof oxidatio i orreduc eddin 2 situ by a supplementary reducing agent, such asbisulfites, thiosulfates, sulfinic acids, benzoin, l-ascorbic acid,primary, secondary and tertiary amines, sodium formaldehyde sulfoxylate,and like reducing compounds.

'The following examples illustrate in detail how the invention iscarried out:

Example 1.-A 325 cc. stainless steel shaker tube was charged with 100cc. of methanol and 1.0 gram of l-hydr'oxycyclohexyl-l-hydroperoxide,commonly known as cyclohexanone peroxide. The tube was then flushed withnitrogen, evacuated to constant pressure to remove the nitrogen andcooled to about 50 C. in a Dry' Ice/ methanol bath. There was then addedto the cold tube 2.0 cc. of 0.090% solution of ferrous chloridetetrahydrate in methanol which is a ferrous ion'concentration based on100 grams of monomer of 5 partsper' million. There was also added 1.0gram of l-ascorbic acid plus 12 cc. of methanol. The tube was flushedwith nitrogen, evacuated, cooled to about '50 C., and charged with 108grams of liquid ethylene. The tube was then placed in a shaker machineand the machine started. After the tube had warmed up to 20 C. it wasremoved from the shakermachine and totally immersed in an ice/ice watermixture and thus maintained at 0 C. for 18 hours. The pressure droppedduring this period from 1410 lb./sq. in. to 1010 lb./ sq. in. Theunreacted ethylene was bled off at 0 C. and the tube opened. Adispersion of ethylene polymer in methanol had formed. The' ethylenepolymer was filtered off and washed first with methanol, their water andfinally with acetone. The yield of solid ethylene polymer was 14.1grams. A hot pressed film'ofthe polymer was hard and stiff. The meltingpoint was 120 C. The moisture permeability value was less'than 10 unitsand the density was 0.9745 g./cc. at 25 C.

' Example 2.A 325 cc. stainless steel shaker tube was charged with 1.0cc. of 0.90% methanol solution of ferrous chloride tetrahydrate, themethanol evaporated oflf with an air stream, and then 1.0 gram "of.l-ascorbic acid and 1.0 gram of 1-hydroxycyclohexyl-l-hydroperoxidewere added. The tube was flushed withnitrogen, evacuated, cooled toabout 50 C., and 173 grams of liquid ethylene was added. The tube waswarmed to 3 C. in a shaker machine and the pressure released to 4500lb./sq. in. whenever it exceeded 4500 lb./sq. in. The tube was thenimmersed in an ice/ice water bath and maintained at 0 C. for 19.5 hours.The pressure dropped from 4500 lb./sq. in. to 4100 lb./sq. in. duringthis period. The unreacted ethylene was then bled 01f at 0 C. and thetube opened. The solid polymer of ethylene was obtained as a flufi whichafter washing with water, methanol, and acetone, possessed a meltingpoint of 118 C. and gave a very stiif film.

Example 3.A 325 cc. stainless steel shaker tube was charged with cc. oftertiary butyl alcohol, 10 cc. methanol, 1.0 gram l-ascorbic acid and1.0gram of l-hydroxycyclohexyl-l-hydroperoxide. The tube wasflushed withnitrogen, evacuated, cooled to about 50 C. and"5.0 cc. of an 0.18%solution of ferrous chloride tetrahydrate in methanol was added. Thetube was again flushed with nitrogen, evacuated, cooled to about '50 C.and grams of liquidethylene was added. The tube was cooled to 80 C. in ashaker box and immersed in anice/ice water bath at 0 C. for 17.5 hours.During this time the pressure ranged from 530to 550 lb./sq. in.Theunreacted ethylene Was bled off at 0 C. A dispersion of ethylenepolymer in alcohol was obtained. The ethylene polymer was washed withwater, methanol, and acetone. The yield was 7.2 grams of solid, powderyethylene polymer. The melting point was 126.5 C. A hotmolded article washard, stiff, glossy, mar-resistant'and possessed a density of 0.980 at25 C. and 'a moisture permeability value of less than 10. When thisethylene p'olymer was applied as a hot melt to paper an adherent coatingwas obtained that was glossy, hard, and mar-resistant. A blend of 20grams of this ethylene polymer with 80 grams of paratfin wax gavecoatings on paper that were glossy and tough as compared to the dull,weak coatings obtained from 100 percent paraffin wax.

Example 4.A 325 cc. stainless steel autoclave was charged with 100 cc.of methanol, 2.0 grams isoascorbic acid and 2.0 grams of1hydroxycyclohexyl-l-hydroperoxide. The autoclave was flushed withnitrogen, evacuated, and cooled to about 50 C. There was then added 2.0cc. of a 0.090% solution of ferrous chloride tetrahydrate in methanol,after which the autoclave was again flushed with nitrogen, evacuated,cooled to about 50 C. and 100 grams of liquid ethylene was added. Thebomb was placed in a shaker box and agitated, while the contents warmedto C. The autoclave was immersed inan ice/ ice water bath and maintainedat 0 C. for 18.5 hours. The autogenous pressure during this periodranged from 730-750 lb./ sq. in. at 0 C. The autoclave was thereafteropened. There was obtained a dispersion of ethylene polymer in methanol.The dispersion was filtered, the ethylene polymer was washed well withmethanol and dried at 70 C. The solid ethylene polymer had a density of0.9931 g./cc. at 25 C.

Example 5.-A 325 cc. stainless steel shaker tube was charged with 44grams of tertiary butyl alcohol and 1.0 cc'. of methyl isobutyl ketoneperoxide. The tube was flushed with nitrogen, evacuated and cooled toabout 50 C. There were then added 6.0 cc. of 0.090% solution of ferrouschloride tetrahydrate in methanol and 1.0 gram of l-ascorbic acid. Thetube was flushed with nitrogen, evacuated, cooled to about 50 C. and 125grams of liquid ethylene was added. The tube was warmed to -2 C. in ashaker machine and immersed in an ice/ ice water mixture at 0 C. for 18hours. The pressure was 1530 lb./ sq. in. Unreacted ethylene was thenbled oif. The ethylene polymer was filtered off, washed well withmethanol, water, methanol, and finally acetone. The solid polymer ofethylene obtained possessed a melting point of 120 C.

Example 6.-A 325 cc. stainless steel tube was charged with 78 cc. ofmethanol, cc. of 0.045% nickelous sulfate hexahydrate in methanolsolution, and 2 grams l-ascorbic acid. The tubewas flushed withnitrogen, evacuated and cooled to about 50 C., 2 grams ofl-hydroxycyclohexyl-lhydroperoxide in 12 cc. of methanol added, afterwhich the tube was again flushed with nitrogen, evacuated, cooled toabout 50 C., and then 100 grams of liquid ethylene was injected into thetube. The tube thus charged was agitated in a shaker box, while thecontents warmed up to 0 C. The tube was then immersed in ice/ice waterand maintained at 0 C. for hours and a pressure of 640-650 lbs./ sq. in.Unreacted ethylene was bled off at 0 C. and the tube opened. There wasobtained a viscous dispersion of ethylene polymer. The polymer wasfiltered off, washed with methanol and dried at 100 C. The solidethylene polymer obtained possessed a density of 0.9762 g./cc. at C.

Example 7.-Example 6 was repeated except that the nickelous sulfate wasreplaced by 0.20 gram of silver nitrate in 10 cc. of methanol. A heavypaste of ethylene polymer was obtained from a reaction conducted at 0 C.and 600-640 lbs/sq. in. ethylene pressure. The polymer had a meltingpoint of 118 C. and moldings were hard and stiff.

Example 8.Example 6 was repeated, except that the nickelous sulfate wasreplaced with 5 cc. of 0.023% man ganese chloride in methanol solution.The ethylene polymer produced under these conditions had a melting pointof 108 C. Example 9.Example 6 was repeated, except that the nickeloussulfate was replaced with 1 cc. of 0.050% cobaltous nitrate hexahydratein methanol solution. The melting point of the solid, hard, stiffethylene polymer produced was 118 C.

Example Ill-A 325 cc. stainless steel shaker tube was charged with 100cc. methanol and 1.0 cc. of tertiary butyl perbenzoate. The tube wasflushed with nitrogen, evacuated and cooled to about C. There was thenadded 2.0 cc. of a 0.090% solution of ferrous chloride tetrahydrate inmethanol, 1.0 gram l-ascorbic acid, and 12 cc. methanol. The tube wasagain flushed with nitrogen, evacuated and cooled to about 50 C., and100 grams of liquid ethylene was added. The tube was agia tated in ashaker box, while the contents warmed to 0 C; The tube was immerced inan ice/ice water bath and maintained at 0 C. for 17 hours. The pressureranged from 560 to 870 lb./ sq. in. Unreacted ethylene was bled off at 0C. and the tube opened. A dispersion of ethyl- I ene polymer in methanolwas obtained. The dispersion was filtered, the ethylene polymer waswashed and dried. The solid ethylene polymer thus obtained had a densityof 0.9737 g./cc. at 25 C. and moldings were hard and stiff. Example11.-A four-liter stirred, stainless steel autoclave was charged with 850grams tertiary butyl alcohol, grams methanol, 10 grams succinic acidperoxide, and 10 grams of l-ascorbic acid. The autoclave was thenevacuated, cooled to 0 C., and 1000 grams of liquid ethylene was addedwith cooling to 0 C. Thereafter there was added at 0 C. 75 grams ofmethanol and 2.0 cc. of a 0.90% solution of ferrous chloridetetrahydrate in methanol. Polymerization was carried out for 4 hours at1-2 C. at a pressure of 550-560 lb./ sq. in. Unreacted ethylene was bledoff and the auto-clave discharged at 0 C. A dispersion of ethylenepolymer was obtained. The polymer was filtered off and washed well withmethanol. The solid ethylene polymer obtained was stiff in the form ofbars and films and melted at 123 C.

Example 12.-A four-liter stirred, stainless steel autoclave was chargedwith 800 grams methanol, 2.0 grams l-ascorbic acid, and 2.0 cc. of a0.90% solution of ferrous chloride tetrahydrate in methanol. Theautoclave was evacuated, flushed with ethylene, evacuated, again cooledto 0 C. and 1000 grams of liquid ethylene was added at 0 C. Thereafter100 cc. of deoxygenated water and 2.0 grams ammonium persulfate wereadded.

Polymerization was carried out for 2% hours at 0 C.-2

C. at an autogenous pressure of 570-600 lb./ sq. in. Unreacted ethylenewas bled off at 0 C. and the autoclave discharged at 0 C. A dispersionof ethylene polymer was obtained. The dispersion was coagulated byaddition of an equal volume of water, filtered, the ethylene polymer waswashed first with water and then with methanol. The solid ethylenepolymer was still in the form of bars and films and possessed a densityof 0.9709 g./ cc. at 25 C.

Example 13.-A 325 cc. stainless steel lined shaker tube was charged withcc. methanol and 1.0 gram of sodium formaldehyde-sulfoxylate dihydrate.The tube was then flushed with nitrogen, evacuated, and cooled to about50 C. There were then added 1.0 cc. of a 0.090% solution of ferrouschloride tetrahydrate in methanol, 9 cc. methanol, and 2.0 gramsl-hydroxycyclohexyll-hydroperoxide. The tube was again flushed withnitrogen, evacuated and cooled to about 50 C., grams of liquid ethylenewas added, the tube warmed to 0 C. in a shaker machine, and immersed inan ice/ice water bath at 0 C. for 18 hours. The pressure ranged from620-650 lb./ sq. in. during this period. The ethylene polymer dispersionformed was filtered, and the ethylene polymer washed first withmethanol, then with water, and finally with methanol. The solid polymerobtained was stiif in the form of bars and films and possessed a densityof 0.9858 g./cc. at 25 C.

Example 14.-A solution of 4 grams of benzoyl peroxide and 0.15 gram offerric acetylacetonate in 100 ml. of thiophene-free benzene was chargedinto a 1600 ml. stainless steel autoclave. A test tube containing 4grams of triethanolamiue was suspended in the autoclave so that itscontents would be emptied when the autoclave was rocked. The autoclavewas flushed three times with nitrogen, cooled in Dry Ice, evacuated andcharged with 280 grams of liquid ethylene. The autoclave was thenbrought to a temperature of C. and rocked for 20 hours. The reactionmixture was discharged and the ethylene polymer removed by filtration.The melting point of the polymer was 120 C.

Example ]5..A solution of 4 grams of benzoyl peroxide in 150 ml. ofthiophene-free benzene was charged into a 1600 ml. stainless steelautoclave. A solution of 2.5 grams of benzenesulfinic acid in 40 ml. ofmethanol was added, and 0.1 gram of ferrous chloride in 10 ml. ofmethanol in .a test tube was suspended in the autoclave so that thecontents of the test tube would be discharged .on rocking. Two hundredgrams of liquid ethylene was charged into the autoclave in the mannerdescribed above, and the autoclave rocked for 20 hours at 0 C. Theproduct was a white powder having a density of 1.096 g./cc. at 25 C.

Example 16.A mixture of 5.0 cc. of thiophene-free benzene and 330 cc. ofmethanol was charged into a 1600 cc. stainless steel autoclave and asolution of 2 grams of ammonium persulfate in cc. of water and 5 cc. ofmethanol was added. A test tube containing 2 grams of sodium bisulfite,0.002 gram of ferrous ammonium sulfate, 5 cc. of water and 5 cc. ofmethanol was suspended in such a manner that rocking the autoclave woulddischarge the contents of the tube. Two

hundred grams of oxygen-free liquid ethylene was charged in the mannerdescribed above and the autoclave rocked at 0 C. for 20 hours andethylene polymer was isolated as a white powder.

Example 17.A solution of 5 cc. of dibutyl zinc in 75 cc. ,of benzene and25 cc. of methanol was charged into a 1600 cc. stainless steel autoclavewhich had previously been flushed with nitrogen. A test tube containing4 grams of powdered, hydrated cupric sulfate was suspended in theautoclave in such a manner that rocking would discharge its contents.The autoclave was charged with 200 grams of liquid ethylene in themanner described above and then rocked at 0 C. for 20 hours. Unreactedethylene was bled ofif and steam was blown through the reaction mixtureuntil the benzene and methanol had been removed. A small amount ofnitric acid was then added to dissolve the zinc and copper salts. Thewhite solid which remained was washed with water, methanol and acetone,then air-dried to give a fluffy white powder. This polymer had a densityof 0.965 g./ cc. at 25 C. The bending modulus of hot pressed films was113,000 lbs/sq. in.

Example 18.A 325 cc. stainless steel tube was charged with 95 cc.methanol plus 1.0 gram sodium formaldehyde sulfoxylate and 2.0 cc. of0.090% ferrous chloride tetrahydrate in methanol solution. The tube wasflushed with nitrogen, evacuated, cooled to about 50 C. and 2 cc. oftertiary butyl hydroperoxide in 5 cc. of methanol added. The tube wasagain flushed with nitrogen, evacuated, cooled to about 50" C. and 100grams of ethylene condensed within the tube. The tube and contents wereagitated in a shaker box while the contents warmed up to 0 C. and werethen immersed in ice/ice water and maintained at 0 C. for 18.5 hours.The pressure during this period was 620-660 lb./sq. in. Unreactedethylene was bled ofi at 0 C. and the tube opened. The ethylene polymerdispersion was filtered oif, washed well with methanol and dried. Thedensity of this ethylene polymer was 0.9944 g./cc. at 25 C.

Example 19.-To a 400 cc. stainless steel vessel was added 40 cc.methanol, cc. of water, and after cooling :to C., 1 gram of potassiumazodisulfonate. The vessel was sealed, evacuated, and cooled to .80 C.and 150 grams of liquid ethylene was bled in. The vessel was immersed inwater at 0 C. for 24 hours, with the .cgntepts agitated by slowlyrotating the vessel end over .end. Un eacted ethylene was discharged andthe tube was opened. A solid material amounting to 1.2 grams wascollected. This product was waxy, insoluble in acetone or cold xylenebut soluble in hot xylene.

Example 20.--A 400 cc. silver-lined vessel to which had been added 0.5gram nickel-on-kieselguhr catalyst was dried by heating several hours atC. under a pressure of 0.5 mercury. The vessel was evacuated and 150 cc.benzene containing 5 grams of lithium butyl was added under anhydrousconditions. The vessel was pressured with 150 grams of ethylene. Thevessel and contents were rotated slowly end over end for 9.5 hours at 0C. Unreacted ethylene was bled oil. The solid polymer which formed waswashed with water, dried and dissolved in hot xylene. The lattersolution was added with stirring to an excess of methanol and theethylene polymer was recovered and dried. The solid polymer melted-at128.4 C., as determined by observing the disappearance of sperulites ona hot stage microscope.

Example 21.--To a glass-lined vessel was added 2 grams ofN-nitrosoacetanilide (prepared according to Johnson and coworkers, J.Am. Chem. Soc. 65, 24-46 (1943)) and 2 grams of dry thiophene-freebenzene. The vessel was evacuated, cooled in liquid nitrogen andethylene distilled in until the vessel was about one-third full ofliquid ethylone (about cc.). The reaction mixture was maintained at 0 C.for 11 days. The vessel was opened and unreacted ethylene allowed toescape. The solid material which was adhering to the walls of thereaction vessel was washed with acetone and dried. Five grams of waxysolid was collected which melted at 119 C. (hot stage microscope). Itwas insoluble in cold xylene but dissolved on heating in this solvent.

The polymerization of ethylene can be carried out in liquid ethylene asthe sole reaction medium or in the presence of an organic medium whichremains liquid below the critical temperature of ethylene (9.6" C.).Typical of such liquids are methanol, tertiary butanol, isooctane,toluene, xylene, and combinations thereof. Mixtures of water and organicliquids which are water soluble can also be used, if desired. Preferredreaction media are methanol, tertiary butanol, and benzene.

Emulsifying agents can be included in the reaction mixture, if desired,and examples are the potassium and sodium salts of long chain aliphaticcarboxylic acids, the sodium and potassium salts of long chain alcoholsulfates or sulfonates, neutral agents such as the polyethylene oxidecondensates, and quaternary ammonium salts, as well as other emulsifyingagents common to the art.

The pH of the reaction medium may be varied Within wide limits,depending upon the system used.

The temperature of the polymerization may be varied from the criticaltemperature, which is 9.6 C. for ethylene, to temperatures of -50 C. orlower, the essential feature being when operating in this lowtemperature range that the ethylene be present as a liquid phase so thatonly nominal pressures are required to achieve a satisfactory monomerdensity that will lead toa high molecular weight, solid polymer ofethylene on polymerization. The pressures to be employed depend upon thenature of the polymerization medium and the degree of polymenizationdesired but must be suflicient to insure that the ethylene be present asa liquid phase with none, or at most an inconsequential part, present asa vapor phase. Pressures in the range of 10 to 100 atmospheres arenormally sufficient. Higher pressures, e. g., u p to 2000 atmospheresmay, however, be used.

The examples illustrate a number of methods in which highly effectivecatalysts for the polymerization of ethylene are used. Some of thesemethods involve a system in which a peroxygen compound is dissociated inthe presence of a polyvalent heavy ion in a lower valence state. Theheavy metal ion is oxidized to its higher valence state and theperoxygen compound is reduced.

7 The presence of the heavy metal is not critical for'operativeness butits use in combination with peroxygen compounds constitutes a preferredmode of operation.

A preferred method of producing a reduction-oxidation catalyst forconducting polymerizations in accord with the invention has beendescribed, generally there being used in such a method a polyvalentheavy metal ion, an oxidizing agent and for optimum results a reducingagent to maintain the metal ion in the reduced state.

Examples of suitable oxidizing agents which also function as freeradical producers include the peroxygen compounds, e. g., the saltsothydrogen peroxide, perborates, percarbonates, persulfates,perphosphates, percarboxylates; organic hydroperoxides such as methylhydropcroxide, ethyl hydroperoxide, 'tertiary butyl hydroperoxide,tetraliu hydroperoxide, cumene hydroperoxide, l-hydroxycyclohexylhydroperoxide-l, and numerous hydroperoxides obtained by adding one moleof hydrogen peroxide to a carbonyl group to obtain the grouping OOHdiacylperoxides such as benzoyl peroxide, acetyl peroxide, acetylbenzoyl peroxide, lauroyl peroxide, trichloroacetyl peroxide, crotonylperoxide, etc.; alkyl acyl peroxides such as tertiary butyl perbenzoate,ditertiary butyl perphthalate, tertiary butyl permaleic acid, tertiarybutyl perphathalic acid; hydrogen peroxide, peracetic acid, perbenzoicacid, di-sobutylene ozonide, methyl ethyl ketone peroxide,acetone-methyl isobutyl ketone peroxide, succinic acid peroxide, methylisobutyl ketone peroxide, dibenzal diperoxide, polyperoxides, diethylperoxidicarbonate, isopropyl perearbonate, pelargonyl peroxide and likematerials. Amounts used are in the range of 0.005 to 3% by weight basedon monomer.

The amount of heavy metal ion added to the polymerization mixture can bemarkedly lowered by the addition of an organic reducing agent whichpossesses the ability to reduce the -ic ion to the -ous ion, thusrenewing the supply of -ous ion when the -ous ion is oxidized to the -icion by the peroxy compound. Under these conditions the amount of ferrousion present, for example, is preferably in the range of 1-1000 parts permillion based on the total amount of polymerizable monomer present. Therate of polymerization is markedly influenced by the amount of ferrousion present with 100 parts per million giving much faster rates thanparts per million. by introduction of a simple or complex salt orcompound in which the metal is present in the -ic state provided that asuitable reducing agent is present to reduce the -ie ,to the -ous ion.Examples of such reducing agents are manifold and include such compoundsas l-ascorbic acid,

d-ascorbic acid, sodium formaldehyde sulfoxylate, dihydroxymaleic acids,formarnidine sulfinic acid, butyraldehyde, sorbose, levulose, inosose,fructose and glucose. These reducing agents are generally used inamounts of 0.005 to 3% based on the total amount of monomer present.

Aliphatic azo compounds operable in the practice of this invention arethose which have an acyclic azo, --N=N, group and which decompose toyield free radicals below 9.6 C. Examples are alpha, alpha-azo-.diisobutyric acid, alpha, alpha-azobis(alpha,gammadimethyl-gamma-methoxyvaleronitrile), alpha, alphaazobis(alpha,gamma-dimethyl-gamma-ethoxyvaleronitrile) alpha,alpha'-azobisalpha, gamma-dimethyl-gammabutoxyvaleronitrile) alpha,alpha"azobisalpha, gamma gamma tnimethylvaleronitrile), alpha,alpha azobis(alpha,gamma dime-thyl gamma phenylvaleronitrile), alpha,alpha'-azobisalpha-phenylpropionitrile) potassium azodisulfonate, and the like. Thesecompounds may be prepared by theprocedure described in U. S. Patent Theheavy metal ion can also be obtained r 2,469,358, issued May 10, 1949,to W. L. Alderson and J. A. Robertson.

The processes of the invention are directed to the polymerization ofethylene with itself, a process called homopolymerization by thoseskilled in this art. It is understood therefore that in thisspecification and in the attached claims references to polymerization,polymers of ethylene and like terms will means homopolymerization andhomopolymers of ethylene and will not be given the generic connotationto include the polymerization of ethylene with other compounds that willpolymerize with it.

The polymerization by any of the processes described may be carried outbatchwise, continuously or semi-continuously. Agitation may be provided,such as by the use of stirring in an autoclave or shaking machines (inbatchwise reactions) or by devices providing turbulent flow in reactorshaving a high ratio of length to crosssection.

The ethylene polymers produced by the process of this invention aremarkedly different in physical properties from ethylene polymersobtained by polymerizing gaseous ethylene under high pressures, e. g.,1500 atmospheres. The difference is one of kind rather than one ofdegree as is demonstrated by the fact that the moisture permeabilityvalues are not additive when the polymers are blended together.Attention is directed to the drawing which illustrates the difference inkind which exists between the polymers of ethylene known to the art andthe polymers of ethylene prepared according to this invention.

In the drawing, graphically shown by curve I is represented the moisturepermeability of a blend of liquid phase ethylene polymer and vapor phaseethylene polymer plotted against percent composition. The ordinate isdivided in moisture permeability units, the values given representingthe grams of water transmitted per hour at a temperature of 39.6 C. per100 sq. meters of surface area for film l-mil thick with 100% relativehumidity on one side of the film and zero percent relative humidity onthe other side. The abscissa is divided in percentage com position byweight of the blended mixture. The blends were made from an ethylenepolymer having a density of 0.9137 g./cc. at C. prepared from gaseousethylene at elevated temperatures and superatmospheric pressures and aliquid phase ethylene polymer produced in accord with the process of theinstant case, having a density of 0.9757 g./cc. at 25 C.

The straight line of the drawing 11 represents the moisture permeabilityvalue that would be obtained if the properties of the blended polymerswere additive using the moisture permeability value of the liquid phasepolymer as substantially zero.

From curve I it will be noted, inter alia, that the addition of 20% ofthe ethylene polymer obtained by polymerizing liquid ethylene below itscritical temperature to 80% of ethylene polymer obtained by polymerizinggaseous ethylene at elevated temperatures and pressures, reduced by themoisture permeability of the latter polymer whereas if the blendproperties were additive the moisture permeability could only have beenreduced by 20%. This constitutes a factual demonstration that these twopolymers must necessarily possess entirely dilferent structures althoughthey are both prepared from ethylene.

A further and outstanding difference between the liquid phase polymersand the vapor phase polymers of ethylene is demonstrated by thedifierence in their Youngs bending modulus. Films of the polymer fromgaseous ethylene exhibited a modulus from 14,000 to 24,000 p. s. i.compared to a modulus of 100,000 to 200,000 p. s. i. for films of thepolymer from liquid ethylene.

The difierence between these polymers is likewise shown by the fact thatthe density values are not additive for a /30 mixture of ethylenepolymer made by polymerizing liquid ethylene below its criticaltemperature with ethylene polymer made by polymerizing gaseous ethyleneat elevated temperatures and pressures (densities 0.9137/0.9757). Thismixture possesses a density of 0.9335 whereas by additive calculationthe density should be 0.9323.

The very high stillness and the outstanding moisture impermeability offilms of the polymer obtained by polymerizing liquid ethylene below thecritical temperature makes it eminently suitable for plastic outletsrequiring good rigidity such as synthetic fibers, monofils, piping,electrical insulation and many kinds of fabricated articles. Theoutstanding moisture impermeability of these ethylene polymers makes thepolymer obtained by polymerizing liquid ethylene well suited as aprotective wrap for foodstuffs, cigarettes, baked goods and the like.

We claim:

1. In a process for the polymerization of liquid ethylene to solidpolymers, the steps which comprise cooling ethylene to below 9.6 C. at asufiicient pressure to produce liquid ethylene and polymerizing theliquid ethylene, the polymerization being catalyzed by the free radicalsobtained from the class of compounds consisting of azo compounds, metalalkyl compounds and peroxygen compounds that give free radicals below9.6 C.

2. The process of claim 1 conducted in the presence of an inert liquidreaction medium.

3. In the process for the polymerization of liquid ethylene, the stepswhich comprise cooling ethylene at least as low as the criticaltemperature, at a suflicient pressure to produce liquid ethylene andpolymerizing the liquid ethylene, in the presence of an active freeradical produced by a metal alkyl compound in conjunction with amaterial selected from the group consisting of the metals of groups 113and VIII of the periodic table.

4. In a process for the polymerization of liquid ethylene, the stepswhich comprise cooling ethylene at least as low as the criticaltemperature, at a suflicient pressure to produce liquid ethylene andpolymerizing the liquid ethylene with the aid of lithium butyl.

5. In a process for the polymerization of liquid ethylene, the stepswhich comprise cooling ethylene at least as low as the criticaltemperature, at a sulficient pressure to produce liquid ethylene andpolymerizing the ethylene in the liquid phase in the presence of a freeradical producing catalyst, at a temperature below 9.6 C., the catalystcomprising a peroxygen compound and an ion of the group consisting ofpolyvalent metal ions in one of their lower valence states and a silverion.

6. The process of claim 5 in which the metal ion is a ferrous ion.

7. The process of claim 5 in which the peroxygen compound isl-hydroxycyclohexyl-l-hydroperoxide.

8. The process of claim 5 in which a reducing agent is present that iscapable of reducing the polyvalent metal -ic ion to a polyvalent metal-ous ion.

9. The process of claim 5 in which the is l-ascorbic acid.

10. In a process for the polymerization of liquid ethylene, the stepwhich comprises polymerizing liquid ethylene at a temperature below 9.6C., in the presence of from 0.005 to 3% by weight of a free radicalproducing catalyst comprising a peroxygen compound and a polyvalentmetal ion in one of its lower valence states.

11. In a process for the polymerization of liquid ethylene, the stepwhich comprises polymerizing liquid ethylene at a temperature below 9.6C., in the presence,

reducing agent based on the total weight of ethylene present, of from0.005 to 3% by weight of a peroxygen compounds and from 0.005 to 3% byweight of a reducing agent that will reduce a ferric ion to a ferrousion.

12. In a process for the polymerization of liquid ethlyene, the stepwhich comprises polymerizing liquid ethylene at a temperature below 9.6C., in the presence of from 0.005 to 3% by weight ofl-hydroxycyclohexyll-hydroperoxide and ferrous ions.

13. In a process for the polymerization of liquid ethylene, the stepwhich comprises polymerizing liquid ethylene at a temperature below 9.6C., in the presence of from 0.005 to 3% by weight ofl-hydroxycyclohexyll-hydroperoxide, of from 0.005 to 3% by weight oflascorbic acid, and ferrous ions.

14. In a process for the polymerization of liquid ethylene, the stepswhich comprise cooling ethylene at least as low as its criticaltemperature, at a sufficient pressure to produce liquid ethylene, andpolymerizing, at a temperature below 9.6 C., the ethylene in the liquidphase with, as the reaction medium, liquid ethylene and an inert organiccompound that is liquid under polymerization conditions, thepolymerization being conducted in the presence of a peroxygen compoundand an ion of the group consisting of polyvalent metal ions in one oftheir lower valence states and a silver ion.

15. In a process for the polymerization of liquid ethylene, the stepswhich comprise cooling ethylene. at least as low as its criticaltemperature, at a suflicicnt pressure to produce liquid ethylene, andpolymerizing, at a temperature below 9.6 C., the ethylene in the liquidphase with, as the reaction medium, liquid ethylene and a solvent of thegroup consisting of methanol, benzene and tertiary butanol, thepolymerization being conducted in the presence of a peroxygen compoundand an ion of the group consisting of polyvalent metal ions in one oftheir lower valence states and a silver ion.

16. In a process for the polymerization of liquid ethylene, the stepswhich comprise cooling ethylene at least as low as the criticaltemperature at a suflicient pressure to produce liquid ethylene andpolymerizing the liquid ethylene in the liquid phase with liquidethylene as the reaction medium and in the absence of ultra violetlight, in the presence of an active free radical produced by a metalalkyl in conjunction with a material selected from the group consistingof the metals of groups IB and VIII of the periodic table and theirsalts.

17. In a process for the polymerization of liquid ethylene, the stepswhich comprise cooling ethylene at least as low as the criticaltemperature at a suflicient pressure to produce liquid ethylene andpolymerizing the liquid ethylene in the liquid phase with liquidethylene as the reaction medium and in the absence of ultra violetlight, with the aid of lithium butyl.

18. In the polymerization of ethylene to solid homopolymers in thepresence of a catalyst effective therefor, the improvement whichcomprises cooling ethylene to a temperature which is at least as low asits critical temperature, at a pressure suflicient to produce liquidethylene, and polymerizing the ethylene in the liquid phase.

References Cited in the file of this patent UNITED STATES PATENTS

18. IN THE POLYMERIZATION OF ETHYLENE TO SOLID HOMOPOLYMERS IN THEPRESENCE OF A CATALYST EFFECTIVE THEREFOR,